1. Field of the Invention
The present invention relates to a Group III nitride compound semiconductor light-emitting element. Particularly, it relates to improvement of a p-side electrode in a Group III nitride compound semiconductor light-emitting element provided with the p-side electrode and an n-side electrode both formed on one surface side.
2. Description of the Related Art
A so-called flip chip type light-emitting element provided with a p-side electrode and an n-side electrode both formed on one surface side is known as a Group III nitride compound semiconductor light-emitting element. The flip chip type light-emitting element is generally configured as shown in FIG. 4. That is, an n-type semiconductor layer 102, a light-emitting layer 103 and a p-type semiconductor layer 104 are formed successively on a light-transmissive substrate 101 such as a sapphire substrate. A p-side electrode 105 and a protective film 106 are formed on the p-type semiconductor layer 104. An n-side electrode 107 is formed on the n-type semiconductor layer 102. Light emitted from the light-emitting layer 103 is radiated out through the substrate 101. On this occasion, apart of light emitted from the light-emitting layer 103 moves toward the electrode side. To improve external radiation efficiency (luminance), it is therefore preferable that the part of light moving toward the electrode side is efficiently reflected by the p-side electrode 105 so as to be used as external radiating light. Hence, there has been a proposal for a configuration in which Ag (or Ag alloy) exhibiting high reflectance to the light emitted from a Group III nitride compound semiconductor light-emitting element is used as the material of the p-side electrode.
When the p-side electrode is made of such an Ag-based material, a high-luminance light-emitting element can be achieved but there arises a problem in reliability. That is, when the Ag-based material is used, there arises a problem that lowering of intensity of emitted light or reduction of the lifetime is caused by migration of Ag.
In the related-art example shown in FIG. 4, Ag in the p-side electrode 105 is distributed all over the p-side electrode 105 at a point of time when the protective film 106 is formed by heating after the p-side electrode 105 is formed. Further, a window 108 is provided in the protective film 106 to secure a bonding region. Accordingly, a region in which Ag is substantially exposed is present, so that migration of Ag occurs easily. On the other hand, another configuration (FIG. 5) has been disclosed in Unexamined Japanese Patent Publication No. Hei. 11-220171. In the disclosed configuration, a p-side electrode 111 made of an Ag-based material is covered with a metal layer 112 not containing Ag. A protective film 113 is further formed on the metal layer 112. Hence, the disclosed configuration can be expected to considerably suppress migration of Ag from the p-side electrode 111. A plasma CVD method in a high-temperature state is, however, used exclusively for forming a protective film 113 good in passivation. By this process history, Ag in the p-side electrode 111 is diffused into the metal layer 112 not containing Ag and provided just above the p-side electrode 111. For this reason, similarly to the configuration shown in FIG. 4, migration of Ag cannot be suppressed to a practical level because Ag is diffused into the metal layer surface where the window 114 of the protective film 113 is formed although the Ag concentration is low.
The invention is designed to solve the problem and an object of the invention is to provide a Group III nitride compound semiconductor light-emitting element with both high luminance and high reliability achieved by suppressing migration of Ag in a p-side electrode.
To achieve the foregoing object, the invention is configured as follows. That is,
a Group III nitride compound semiconductor light-emitting element is provided with a p-side electrode and an n-side electrode both formed on one surface side, wherein the p-side electrode includes: a first metal layer containing Ag and formed on a p-type semiconductor layer; an electrically insulating protective film with which the first metal layer except a part region is covered; and a second metal layer not containing Ag and formed on the protective film.
According to this configuration, first, the light-emitting element can be provided as a light-emitting element high in external radiation efficiency because the use of the first metal layer containing Ag makes the semiconductor layer side surface of the p-side electrode have high reflectance so that light emitted from the light-emitting layer can be efficiently reflected by this surface. Moreover, migration of Ag from the first metal layer to the second metal layer can be suppressed effectively because the first metal layer and the second metal layer are electrically insulated from each other by the protective film except a part region. In other words, migration of Ag from the first metal layer to the p-side electrode surface (that is, the surface of the second metal layer) can be suppressed greatly, so that a highly reliable light-emitting element can be formed. On the other hand, electrical contact between the first metal layer and the second metal layer can be still sustained by the part region which is not shielded by the protective film. In this manner, a light-emitting element with both high luminous intensity and high reliability can be provided according to the configuration.
Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.